The present invention relates to an optical disc apparatus which reads data recorded on an optical disc such as a CD or a DVD, and which outputs a reproduced signal based on the read data, and a method for controlling focus thereof.
In an optical disc apparatus which reads data recorded on an optical disc such as a CD or a DVD, and which outputs a reproduced signal (an audio signal and a video signal), conventionally, a focus control is performed to control the focus of a laser beam irradiating the optical disc.
In a usual optical disc apparatus, a four-split photodetector in which two photodetector elements are arranged in each of the vertical and lateral directions is disposed on a pickup head, and the focus control is performed by using a differential signal (hereinafter, referred to as focus error signal) indicative of a difference between a sum of amounts of reflected light detected by two of upper right and lower left ones of the photodetector elements, and that of amounts of reflected light detected by two of upper left and lower right ones of the photodetector elements. As well known in the art, the focus error signal is a signal which has an S shape characteristic shown in FIG. 7A with respect to deviation in focus on an optical disc. In the figure, the abscissa shows the displacement distance of an objective lens, and the lens is closer to an optical disc as further moving toward the right side.
In a process of reading data from an optical disc, a higher reading accuracy is attained as RF is larger. Therefore, a focus control in a usual optical disc apparatus is performed in the following manner.
In a focus control, a point which is between the points A and B shown in FIG. 7A, and at which the amount of reflected light (RF which is a sum of outputs of the four photodetector elements) reflected from the optical disc and detected by the photodetector is at the maximum is set as a reference point (Vref). The objective lens is displaced so that the level of the detected focus error signal coincides with the reference point. FIG. 8 is a view illustrating a displacement of an amount of light reflected from a disc. In FIG. 8, an abscissa axis indicates a displacement distance of an object lens as in FIG. 7, and an ordinate axis indicates a sum signal of outputs from the four-split photodetector elements.
Ideally, the reference point should be the center O between the points A and B in the focus error signal (the state of FIG. 7A is an ideal one). Actually, however, the reference point is deviated from the center O because of accuracies of optical components and the like constituting the pickup head (see FIGS. 7B and 7C), and hence the focus error signal is not well balanced. When a focus control is performed by using the focus error signal shown in FIG. 7B or 7C, an out-of-focus condition is easily produced (a state where the objective lens is moved to a position where the focus error signal cannot be obtained easily arises), thereby causing a problem in that the focus control is unstable. Namely, if a focus error signal of which output characteristics is shown in FIGS. 7B and 7C is used as it is to start a focus control operation, there are problems that firstly failure of focus-on is easily produced, and that secondly an out-of-focus condition is easily produced. Further, when such an out-of-focus condition is produced during a process of reading data from an optical disc, the objective lens is often largely displaced. This causes another problem in that, depending on the displacement direction of the objective lens, the objective lens collides with the optical disc to be broken.
The Unexamined Japanese Patent Application Publication No.2000-339704 proposes a technique in which, when an out-of-focus condition is produced, a bias is applied so that an objective lens is displaced in a direction opposite to an optical disc, and deviation from the balance of a focus error signal due to the application of the bias is adjusted. However, the publication teaches nothing about a technique of adjusting deviation from balance due to accuracies of optical components and the like constituting a pickup head.